CHAPTER 



4 



The Availability of Soil 

 Anions 



ROY OVERSTREET AND I. A. DEAN 



T 



he availability of soil anions might well be considered 

 as the state of being sufficient for the use of plants. Such a definition 

 cannot readily be given a quantitative interpretation. Mineral nutrient 

 availability usually embraces an integration of factors influencing the 

 absorption of ions from soils and most studies concerned with availa- 

 bility give consideration to the factors which influence it. For example, 

 organic matter and liming are believed to increase while fixation and 

 lack of movement of phosphate ions tend to reduce phosphate availa- 

 bility. Indices of availability vary widely. Measurements such as the 

 total amounts absorbed during the crop season and rates of absorption 

 over a short period of time have been used. 



Nitrates, sulfates, phosphates, borates, and molybdates are soil anions 

 essential for plant growth. In addition, plant ash contains chlorides and 

 silicates in important quantities. Available anions exist either in the 

 soil solution or in a solid phase in equilibrium with this solution. This 

 system contains — in addition to the anions absorbed by plants — carbon- 

 ate, bicarbonate, hydroxyl, fluoride, humate, and sometimes arsenate 

 and arsenite anions which frequently have an important bearing on the 

 state of equilibrium of a soil water system. 



By and large, consideration of anion availability has been restricted 

 to a given plant nutrient. This discussion is an attempt to bring together 

 the factors involved with the availability of soil anions in general. 



ORIGIN AND DISTRIBUTION OF SOIL ANIONS 



The essential elements absorbed by plants in the form of anions exist 

 in soils in several discernible forms. These will be discussed under 

 several different headings. 



